Structure Of CdSe Research Articles

Effect of lateral size and thickness on the electronic structure and optical properties of quasi two-dimensional CdSe and CdS nanoplatelets

The effect of lateral size and vertical thickness of CdSe and CdS nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an effective-mass envelope function theory based on the 8-band k ⋅ p model with valence force field considerations. Volumetrically larger NPLs have lower photon emission energy due to limited quantum confinement, but a greater transition matrix element (TME) due to larger electron-hole wavefunction overlap. The optical gain characteristics depend on several factors such as TME, Fermi factor, carrier density, NPL dimensions, material composition, and dephasing rate. There is a red shift in the peak position, more so with an increase in thickness than lateral size. For an increasing carrier density, the gain spectrum undergoes a slight blue shift due to band filling effect. For a fixed carrier density, the Fermi factor is higher for volumetrically larger NPLs and so is the difference between the quasi-Fermi level separation and the effective bandgap. The transparency injection carrier density (and thus input current density threshold) is dimension dependent and falls for volumetrically larger NPLs, as they can attain the requisite exciton count for transparency with a relatively lower density. Between CdSe and CdS, CdSe has lower emission energy due to smaller bandgap, but a higher TME due to lower effective mass. CdS, however, has a higher so hole contribution due to a lower spin-orbit splitting energy. Both CdSe and CdS NPLs are suitable candidates for short-wavelength LEDs and lasers in the visible spectrum, but CdSe is expected to exhibit better optical performance.

Inverted organic solar cells using a solution-processed TiO2/CdSe electron transport layer to improve performance

In the present work, cadmium selenide (CdSe) nanoparticles are deposited directly on TiO2 film to fabricate the TiO2/CdSe interlayer by a chemical bath deposition method. The inverted organic solar cells using poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction as an active layer and TiO2/CdSe interlayer as an electron transport layer (ETL) are fabricated in air. A series of microstructural, photo-electronic, and electrochemical characterizations on these cells are performed. The TiO2/CdSe structure with respect to either the TiO2 layer or the CdSe layer as the ETL exhibits significantly enhanced external quantum efficiency (EQE) in the visible region. The photoluminescence (PL) measurement shows that the exciton dissociation in the TiO2/CdSe structure is more effective than that in either the TiO2 or CdSe structure. The Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) implies that the charge recombination in the TiO2/CdSe structure can be suppressed with respect to that in either the CdSe or TiO2 structure. The photovoltaic performances of the cells with the TiO2/CdSe ETL are clearly improved compared with the reference cells only with the TiO2 layer or CdSe layer as the ETL.

Preparation and Study of the Fluorescence Quenching for Different Core/Shell Structures Quantum Dots with Mercury (II) Induction

The probes and sensors of the semiconductor quantum dots are the topic of intense research because they are able to show high-sensitivity fluorescence quenching under different ions conditions. Herein, different multishell structures of CdSe, CdSe-CdS, and CdSe-CdS-ZnS water-soluble quantum dots have been synthesized in aqueous solution. Transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray powder diffraction indicate that these quantum dots have high stability, homogeneous particle size and good monodispersity. More precisely, with different concentrations of mercury (II) induction, the mechanism of the fluorescence quenching for various structures of the quantum dots is discussed based electron transfer and nonradiative recombination patterns. Obtained results indicate that the core-shell structure of the CdSe-CdS quantum dots has the most beautiful performance, which shows high sensitivity in the detection of mercury (II). Moreover, this work provides some guidelines for the rational design of others fluorescence bioprobes and biosensors.

Sol gel synthesis of SnO2/CdSe nanocomposites and their optical structural and morphological characterizations

Abstract Optically important SnO 2 /CdSe Nanocomposites were synthesized by low cost ultrasonic assisted sol–gel method. Sol was prepared in ultrasonic environment at 70 °C temperature using Tin chloride, cadmium chloride and selenium dioxide in ethylene glycol. The films were prepared on cellulose acetate flexible substrates by spin coating method. The structural, morphological and optical studies were characterized by XRD, AFM, SEM, Photoluminescence studies (PL) and UV–visible techniques. Shifting in XRD peaks to larger angle indicates lattice strain occurred between SnO 2 and CdSe structures. AFM and SEM micrographs also show the conversion of SnO 2 hollow nanostructures into flake like structures. Shifting of Photoluminescence peaks towards higher wavelength and origin of new absorption band in visible region for SnO 2 /CdSe as compared to core SnO 2 observed due to efficient charge separation occurs in electronic levels in the nanocomposites make them suitable for various optoelectronic applications.

(Invited) Topological States in Multi-Orbital Honeycomb Lattices of HgTe (CdTe) Quantum Dots

Recent advancements in colloidal chemistry demonstrate that two-dimensional single-crystalline sheets of semiconductors forming a honeycomb lattice can be synthesized by oriented attachment of semiconductor nanocrystals [1,2]. Inspired by these results, we have performed atomistic tight-binding calculations of the band structure of CdSe [3,4] and HgTe [5] sheets with honeycomb nano-geometry. We have also considered honeycomb super-lattices of quantum dots that could be made using nano-lithography of HgTe layers.In the case of CdSe sheets [3], we predicted that their conduction band exhibits Dirac cones at two distinct energies and nontrivial flat bands. The lowest Dirac conduction band has s-orbital character and is equivalent to the π bands of graphene but with renormalized couplings. The conduction bands higher in energy have no counterpart in graphene; they combine a Dirac cone and flat bands because of their p-orbital character. We also present very recent results on HgTe [5]. We show theoretically that honeycomb lattices of HgTe can combine the effects of the honeycomb geometry and strong spin-orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin-orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of HgTe quantum dots constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase. [1] W. H. Evers, B. Goris, S. Bals, M. Casavola, J. de Graaf, R. van Roij, M. Dijkstra, and D. Vanmaekelbergh, Nano Lett. 13, 2317 (2013). [2] M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. T. Rabouw, S. A. P. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, S. Bals, and D. Vanmaekelbergh, Science 344, 1377-1380 (2014). [3] E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allan, D. Vanmaekelbergh, Phys. Rev. X 4, 011010 (2014). [4] C. Delerue, Phys. Chem. Chem. Phys., 2014, doi: 10.1039/C4CP01878H. [5] Beugeling, W. et al, Nat. Commun. 6:6316 doi: 10.1038/ncomms7316 (2015).

Electronic band structure of zinc blende CdSe and rock salt PbSe semiconductors with silicene-type honeycomb geometry

We report on the electronic band structure of 2D CdSe and PbSe semiconductors that have a silicene-type honeycomb geometry. Atomistic tight-binding calculations are performed on several model systems that bear a strong resemblance to the silicene-type honeycomb structures that were recently obtained by nanocrystal self-assembly. The calculated band structures are compared both to those of 2D quantum wells and graphene-type honeycomb structures. It is found that in silicene type CdSe honeycomb structures, the lowest electron conduction bands (derived from S-type nanocrystal wave functions) form a Dirac-type dispersion, very similar as in graphene. The P-type bands are usually more complex. However, when the hybridization between S- and P-type bands increases, a second Dirac cone and a genuine non-trivial flat band is observed, similar as in the case of graphene-type honeycomb structures of CdSe. There is a strong non-trivial gap between the first and second valence band, hosting the quantum spin Hall effect. Silicene-type PbSe structures show Dirac features in their bands, which however can be clouded due to the multi-valley character of PbSe.

Structure and Properties of a (CdSe)6@(CdSe)30Cluster Doped with Mn Atoms

The structure and properties of a (CdSe)6@(CdSe)30 cluster doped with one or two Mn atoms were studied using density functional theory. Geometrical structure optimizations were performed using relativistic effective core potentials for Se and Cd atoms and an all-electron description for Mn atoms. The (CdSe)6 core corresponds to a bulk-type structure, and each shell atom is connected to three atoms of the opposite type. We considered different Mn substitutions of a single Se and Cd atom, substitutions of two Cd atoms with two Mn atoms, and additions of one and two Mn atoms to the (CdSe)6@(CdSe)30 cluster. We found that antiferromagnetic singlet and high-spin ferromagnetic states of the Mn2Cd4Se6@(CdSe)30 clusters are nearly degenerate in total energy. In the Cd-substituted clusters, the binding energies of Mn atoms were found to be appreciably larger than the binding energies of Cd atoms in the initial cluster, which explains the high miscibility of Mn in bulk CdSe. Experimental observations of dramatic changes in the magnetic behavior of Mn-doped QDs after annealing can be explained from the results of our calculations of total energy as a function of total spin.

Electronic and structural properties of wurtzite III-V and II-VI structures

The electronic and structural properties of wurtzite InN, ZnS and CdSe structures are studied using the empirical pseudopotential method (EPM). The principal energy at Γ of wurtzite InN, ZnS and CdSe compounds are calculated. Band structure of wurtzite InN, ZnS and CdSe materials are presented. Other quantity such as bulk modules by meansof our model is calculated.

Shell deposition of CdSe nano dots and rods

To investigate the shell deposited kinetics, CdSe quantum dots (QDs) and nanorods (NRs) with a maximum length of 17 nm were fabricated via organic synthesis routes. CdSe with a hexagonal crystal structure (wurtzite) favors epitaxial growth on the {002} surfaces when well-controlled conditions were used. The morphologies and sizes of CdSe samples depended strongly on chemicals and temperature. In the case of 320 °C, CdSe NRs with adjusted length of 7–17 nm were obtained from trioctylphosphine oxide (TOPO) and tetradecylphosphonic acid (TDPA). In contrast, short CdSe NRs (less than 10 nm) were created from octadecylphosphonic acid (ODPA) and trioctylamine (TOA). Spherical CdSe QDs were further fabricated using stearic acid (SA) and TOPO at 300 °C. CdSe cores were coated with Cd0.5Zn0.5S and CdTe shells. Anisotropic growth occurred during shell deposition because CdS shells grown preferentially on the {001} facet of the CdSe core. In the case of CdSe core prepared from TOPO and TDPA, CdSe/Cd0.5Zn0.5S core/shell samples prepared from long CdSe NRs (more than 10 nm) revealed a peanut morphology while the core/shell samples created from short ones (less than 10 nm) exhibited a spherical morphology. All of the CdSe/Cd0.5Zn0.5S core/shell samples revealed a similar length to that of the CdSe cores. This phenomenon was also observed for the core/shell samples fabricated using CdSe NRs prepared by ODPA and TOA. This is ascribed to the well-developed crystal structure of CdSe NRs fabricated using an organic synthesis at high temperature. In contrast, this anisotropic growth did not occur when spherical CdSe QDs prepared from SA and TOPO and the shell (Cd0.5Zn0.5S) coating carried out using SA and TOA. To indicate the shell depositing process, CdSe NRs fabricated using TDPA and TOPO were coated with a CdTe shell. CdTe monomers were deposited on the middle and tip parts of the CdSe NRs to form a tetrapod-like morphology at 220 °C. This is ascribed to the large difference of structure of CdSe (hexagonal) and CdTe (zinc blende).

Preparation and Performance Characterization of “Turnjujube-Like” CdSe Nanofibers

It was reported for the first time that “Turnjujube-like” CdSe nanofibers have been assembled onto TiO2surface by electrochemical method of cyclic voltammetry, using CdSO4•8H2O and SeO2as raw material, can be used as a sensitizer of quantum dot-sensitized solar cell. CdSe morphology, structure characterization and optical properties are characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), and ultraviolt-visible spectroscopy (UV–Vis), respectively. The experimental results show that the prepared “Turnjujube–like” CdSe nanofibers are zinc blende CdSe with cubic crystal system, and it have excellent light absorption within the wavelength range of 300–800 nm. It has a potential for the construction of QD-sensitized solar energy cell.

Investigation on the influence of pH on structure and photoelectrochemical properties of CdSe electrolytically deposited into TiO2 nanotube arrays

In this work, we fabricated CdSe/TiO2 nanotube arrays (NTAs) by electrochemical method. In electrodeposition, the pH value of the electrolyte played an important role in formation of CdSe nanoparticles. As the pH value decreased, more CdSe deposited on TiO2 NTAs. Scanning electron microscopy and transmission electron microscopy characterization shows that the CdSe nanoparticles were uniformly deposited on and into TiO2 nanotubes when the pH value was 3, and this structure fully utilized the three-dimensional (3D) space of TiO2 nanotubes to form 3D multijunction heterostructures. According to the photoelectrochemical test, the CdSe/TiO2 NTAs sample prepared at pH=3 exhibited maximum photocurrent and open circuit potential. This is because that the deposited CdSe nanoparticles had better bond with the walls of TiO2 nanotube than the samples deposited at other pH values, which facilitated the propagation and kinetic separation of photogenerated charges.

Modifications in structural and electrical properties of gamma irradiated CdSe nanowires

Semiconducting nanowires have gained huge interest of researchers due to their interesting properties and potential application in the field of nanoelectronics and optoelectronics. Present study reports the effect of gamma (γ) irradiation on structural and electrical properties of CdSe nanowires. These nanowires were synthesized by a template assisted electrodeposition method which provides uniform and standing nanowires of cylindrical shape. These synthesized nanowires were exposed to gamma rays using a 60Co source. Before and after irradiation, the nanowires were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray (EDX) and Keithley 2400 series source meter to study their structural and electrical properties. XRD patterns of irradiated samples showed increase in grain size with radiation dose and increase in the peak intensity was observed corresponding to different planes of hexagonal CdSe structure. SEM images confirmed the cylindrical morphology of nanowires and EDX analysis revealed the stoichiometric formation of CdSe. The electrical properties from I–V characteristics showed an enhancement in the electrical conductivity of the nanowires with radiation dose and applied potential. This modification in electrical conductivity may be attributed to decrease in grain boundary scattering of charge carriers.

Electrochemical studies of the effects of the size, ligand and composition on the band structures of CdSe, CdTe and their alloy nanocrystals

In this paper, we have elucidated the fundamental principle of employing CV to investigate the band structures of semiconductor nanocrystals (SNCs), and have also built up an optimal protocol for performing such investigation. By utilizing this protocol, we are able to obtain well-defined and characteristic electrochemical redox signals of SNCs, which allows us to intensively explore the influences of the particle size, the surface ligand and particle composition on the band structures of CdSe, CdTe and their alloy nanocrystals. The size-, ligand- and composition-dependent band structures of CdSe and CdTe nanocrystals (NCs) have therefore been mapped out, respectively, which are generally consistent with the previous theoretical and experimental reports. We believe that the optimal protocol and the original results regarding electrochemical characterization of SNCs demonstrated in this paper will definitely benefit the better understanding, modulation and application of the unique electronic and optical properties of SNCs.

Structure and magnetic properties of (CdSe)9 doped with Mn atoms

The electronic and geometrical structures of the (CdSe)9 cluster doped with Mn atoms are studied using density functional theory with generalized gradient approximation. It is found that the Fermi coupling constants in the Mn-substituted CdnSemMnk clusters (k=1 and 2) are very sensitive to the Mn nearest neighbors. Generally, Se neighbors decrease the intensity of the Fermi coupling constant, whereas Cd neighbors amplify it. It is found that the Fermi coupling constant computed for the case when a Mn atom substitutes for a Se atom is close to the experimental value. Mn atoms added to the (CdSe)9 cluster either stick to Se atoms from inside or push out Cd atoms and occupy their previous positions. The lowest total energy state of (CdSe)9Mn is ferromagnetic, the antiferromagnetic singlet state and the ferromagnetic state with the total spin of 5 of (CdSe)9Mn2 are nearly degenerate in total energy, and the lowest total energy state of (CdSe)9Mn3 is ferrimagnetic with the total spin of 2.5. An excited singlet state of (CdSe)9Mn3 is found to possess the largest in the absolute value Fermi coupling constant among all species considered.

Spectroscopy And Structural Study On CdSe Thin Films Deposited By Chemical Bath Deposition

Cadmium selenide (CdSe) thin films were prepared by chemical bath deposition technique on glass substrates using cadmium acetate dihydrate as a source of Cd 2+ ions, and sodium selenosulphate as a source of Se 2- ions respectively. Triethanolamine and ammonia were used as complexing and pH controlling agent. Films were deposited at different bath temperatures ranging from room temperature to 80 o C while the pH value was kept constant around 10.50 ±0.10. We determined elemental composition of deposited film by X-ray photoelectron spectroscopy. Optical properties of films deposited at different bath temperatures are studied by absorption spectra and parameters like thickness, band gap and absorption coefficient were calculated from the optical transmission spectra. The optical spectra showed a small blue shift and the band gap was found to be greater than the bulk phase. Raman measurements performed at room temperature using Helium-neon laser and Argon laser confirmed the presence of longitudinal optical phonon modes. The selected area electron diffraction pattern for CdSe thin films indicated that they are having mixed nature i.e. both single crystalline and polycrystalline nature with planes of diffraction rings corresponding to cubic structure of CdSe respectively.

Hybrid heterostructures based on aromatic polyimide and semiconductor CdSe quantum dots for photovoltaic applications

A nanohybrid photoactive material based on aromatic polyimide (PI) doped with CdSe quantum dots (QDs) has been developed to be used in photovoltaic solar cells. The solar cell is based on a heterostructure of an ITO electrode covered with a layer of Cu–phthalocyanine and a layer of a PI–QD composite. The photovoltaic properties of the CuPc/PI:CdSe hybrid heterostructure at various QD concentrations in the PI matrix have been studied. Luminescent and transmission electron microscopy analyses have shown that the optimal QD mass concentration is 60%. The efficiency of the solar cell based on optimized PI:CdSe structures approaches those for the structures based on conventional MEH-PPV organic semiconductor. Moreover, the photovoltaic characteristics of the solar cell remain stable in the air for a long time (120 h). This is expected to considerably simplify the technology of manufacturing these hybrid solar cells. The mechanisms of the excitation and charge transfer from QDs to the organic semiconductors and influence of QDs on the efficiency of solar energy conversion to electricity are discussed.

CdSe nanofiber and nanohorn structures on ITO substrates fabricated by electrochemical deposition

Self assembled 3-D CdSe structures have been successfully electrodeposited from aqueous solutions. Proposed method allows a control of self-assembled nanostructures formation with a different morphology by varying the composition of the electrolyte in electrochemical bath. The two types of 3-D nanostructures: CdSe nanofibers (ø 30nm, length of 1–2μm) and Se core–CdSe shell nanohorns (ø 200nm, length of 600nm) were fabricated and investigated. The possible mechanism of electrochemical formation of CdSe nanofibers and nanohorns has been proposed.

Quantum confinement effect in multilayer structure of alternate CdSe and SiOx insulator matrix thinfilms

Multilayer (ML) structure of layer-by-layer deposited CdSe/SiOx thin films and their monolayers were prepared using sequential thermal evaporation technique. X-ray diffraction study confirmed the (002) plane of CdSe with wurtzite structure. It is noticed that the microstrain, developed in ML thin films, increased with decreasing particle size. Experimentally measured band gap energies confirmed the splitting of valence band energy levels which rise due to hole confinement in CdSe. Crystallite sizes (5–7nm) were calculated using the effective mass approximation model (i.e., Brus model) which shows that the diameter of crystallites was smaller than the Bohr exciton diameter (11.2nm) of CdSe. The main band in the emission spectra of ML samples gradually shifted to longer wavelength side when particle size was increased from 5 to 7nm. This is characteristic of quantum size effect. It is inferred that disorderliness in CdSe/SiOx ML thin films would increase when the thickness of CdSe sublayer is greater than that of SiOx matrix layer.

Photostability of CdSe-based nanocrystalline structures used to visualize biological tissues

The structure, fluorescence parameters, and photostability of nanocrystalline CdSe and CdSe/ZnS intended for biomedical applications are examined. The photostability of nanocrystalline CdSe/ZnS with different functional and protective coatings is studied. The possibility of using semiconductor nanoparticles as markers in various types of living cells is demonstrated.

Hollow Structure of CdSe by W/O Microemulsion Method

CdSe nanocrystalline is an important Ⅱ - Ⅵ clan semiconductor materials, In this paper the hollow structure CdSe nanoparticles was systhesised in CTAB/cyclohexane/isobutanol/water quatemary microemulsion system. The morphology of the final products were characterized by X-ray power diffraction, transmission electron microscopy and EDS. Results of XRD and EDS revealed that nano-crystals were the mixture of CdSe, TEM images shows nanoball, nanotube and bamboo-like nanotube are hollow structure. This paper proves that the hollow structure CdSe can be obtained in the proper conditons.